1. Field of the Invention
This invention relates to high channel conductivity Nitrogen polar (N-polar) High Electron Mobility Transistors (HEMTs) with (In,Al,Ga)N combination backbarriers, and a method of fabrication thereof.
2. Description of the Related Art
The terms “(In,Al,Ga,B)N,” or “Group III nitride,” or “III-nitride,” or “nitride,” as used herein are equivalent and refer to any alloy composition of semiconductors having the formula GawAlxInyBzN where 0≦w≦1, 0≦x≦1, 0≦y≦1, 0≦z≦1, and w+x+y+z=1. For example, (In,Al,Ga)N could include any alloy composition of semiconductors having the formula InxAlyGazN where 0≦x≦1, 0≦y≦1, 0≦z≦1, and x+y+z=1. Moreover, the use of these terms is intended to be broadly construed to include respective nitrides of the single species, B, In, Al and Ga, as well as binary, ternary and quaternary compositions of such Group III metal species, including, but not limited to, the compositions of AlN, GaN, AlGaN and InAlN. Further, materials within the scope of the invention may further include quantities of dopants, or other impurities, or other inclusional materials, typically in concentrations which are substantially lower than the concentrations of the Group III compounds.
The terms “Nitrogen polar” or “N-polar,” or equivalently “Nitrogen face” or “N-face,” describe the crystallographic nature of the surface of the (Al,Ga,In,B)N structures. Similarly, the terms “Gallium polar,” “Ga-polar,” or “III-polar,” or equivalently “Gallium face,” “Ga-face,” or “III-face,” also describe the crystallographic nature of the surface of the (Al,Ga,In)N structures.
N-polar describes devices wherein the epitaxial layers are epitaxially grown in a [0 0 0 −1] direction of the wurtzite crystal, or on a [0 0 0 −1] surface of a III-nitride layer, where [0 0 0 −1] is an orientation in [h i k l] Miller index notation, and where the top or final grown surface of each layer comprises Nitrogen atoms or a [0 0 0 −1] surface. In N-face devices, the electrodes (e.g., the source, gate, and drain electrodes in the case of transistor devices) are typically formed on a nitrogen face or [0 0 0 −1] face of the underlying III-N material.
The term “semipolar plane” can be used to refer to any plane that cannot be classified as c-plane, a-plane, or m-plane. In crystallographic terms, a semipolar plane would be any plane that has at least two nonzero h, i, or k Miller indices and a nonzero l Miller index. Subsequent semipolar layers are equivalent to one another, so the bulk crystal will have reduced polarization along the growth direction.
These devices are typically grown epitaxially using growth techniques, such as Metal Organic Chemical Vapor Deposition (MOCVD), molecular beam epitaxy (MBE), hydride vapor phase epitaxy (HVPE), chemical beam epitaxy (CBE), etc.
III-Nitride devices such as III-N transistors are typically III-polar devices, such that the electrodes of the device are formed on a Group III face or [0 0 0 1] face of the underlying III-N material. N-polar devices have shown promise in allowing for devices with very low contact resistances. However, further improvements in N-polar devices are needed in order to allow for their widespread adoption.